Apparatus Having Feedback Loops Between Multiple Pairs of Instrumentation Modules

ABSTRACT

The apparatus includes a frame to accommodate more than two instrumentation modules; a multiplexed bus associated with the frame and including more than two connectors; and a sense instrumentation module and a source instrumentation module mounted in the frame, electrically connected to respective ones of the connectors, and collectively including a communication link, and a circuit path that includes, in series between a sense input and a source output, a sense circuit to generate a sense output signal representing a sensed parameter dependent on a source signal; a feedback controller to generate a control signal in response to the sense output signal; and a source circuit to generate the source signal in response to the control signal. The communication link is in the circuit path, and includes a bus transmitter and a bus receiver each configured to select a channel of the multiplexed bus via which to communicate.

BACKGROUND

Some measurement tasks, such as four-terminal (Kelvin) resistancemeasurements and the measurement of the threshold voltage of multipleFETs, require that feedback loops be established between more than onesource circuit and more than one sense circuit. Currently-availablemeasurement equipment, such as the model 4142B modular DC source/monitorformerly sold by Agilent Technologies, Inc., Santa Clara, Calif., canonly establish a feedback loop between a single source circuit and sensecircuit, and is therefore unsuitable for performing measurement tasksthat require multiple feedback loops.

Accordingly, what is needed is a measurement apparatus in whichrespective feedback loops can be established within multiple pairs ofsense circuits and source circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram showing a minimalist example of a measurementapparatus in accordance with the disclosure.

FIG. 1B is a block diagram showing another example of a measurementapparatus in accordance with the disclosure.

FIGS. 2, 3, and 4 are block diagrams showing respective examples of thesense instrumentation module and source instrumentation module of themeasurement apparatus shown in FIG. 1A or FIG. 1B.

FIG. 5 is a schematic drawing showing an example of a multiplexed busimplemented with time-division multiplexing.

FIG. 6 is a block diagram showing an example of a measurement apparatusas disclosed herein configured to perform a four-terminal (Kelvin)resistance measurement.

FIG. 7 is a block diagram showing an example of a measurement apparatusas disclosed herein configured to measure respective threshold voltagesof multiple FETs concurrently.

FIG. 8 is a block diagram showing an example of a measurement apparatusas disclosed herein having a source instrumentation module configured tooperate in response to respective feedback signals received from two ormore sense instrumentation modules.

FIGS. 9A and 9B are block diagrams showing respective examples of asense circuit.

FIGS. 10A and 10B are block diagrams showing respective examples of asource circuit.

FIG. 11A is a block diagram showing an example of a feedback controller.

FIG. 11B is a block diagram showing an example of a feedback controller.

DETAILED DESCRIPTION

The following description refers to signals. In some examples, thesignals are analog signals. In other examples, the signals are digitalsignals. Some examples have both analog signals and digital signals.

FIG. 1A is a block diagram showing a minimalist example of a measurementapparatus 100 in accordance with the disclosure. Measurement apparatus100 includes a frame 102, a multiplexed bus 110, a sense instrumentationmodule 122, and a source instrumentation module 124. Senseinstrumentation module 122 and source instrumentation module 124 arereferred to herein generically as instrumentation modules 120. The terminstrumentation module is abbreviated as I.M. in the drawings. Frame 102is to accommodate more than two instrumentation modules 120. Multiplexedbus 110 is associated with frame 102, and includes connectors 112 toconnect to more than two instrumentation modules 120. In the exampleshown, sense instrumentation module 122 and source instrumentationmodule 124 are mounted in respective slots 104 of frame 102 and areconnected to multiplexed bus 110 by respective connectors 112. Otherways of mounting instrumentation modules 120 in frame 102 and connectingthem to multiplexed bus 110 are known and may be used.

FIG. 1B is a block diagram showing another example of measurementapparatus 100 in accordance with the disclosure. The example shownincludes additional instrumentation modules 126 and 128 mounted in frame102. Each of the additional instrumentation modules is connected by arespective connector 112 to multiplexed bus 110. In the example shown,instrumentation module 126 is a sense module, and instrumentation module128 is a source module. Other examples of measurement apparatus 100include more or fewer instrumentation modules 120 than the examplesshown in FIGS. 1A and 1B. Additionally or alternatively, other examplesof measurement apparatus 100 include numbers of sense instrumentationmodules and source instrumentation modules different from the examplesshown in FIGS. 1A and 1B. In the example shown, sense instrumentationmodule 126 and source instrumentation module 128 are mounted inrespective slots 104 of frame 102. Other ways of mounting additionalinstrumentation modules 120 in frame 102 are known and may be used.

Sense instrumentation module 122 and source instrumentation module 124are linked by a first channel 114 of multiplexed bus 110.Instrumentation modules 120 that are linked by a channel of multiplexedbus 110 are said to be paired. Multiplexed bus 110 has N channels. Thechannel of multiplexed bus 110 used to link two of the instrumentationmodules 120 can be selected from any of the N channels of multiplexedbus 110 that are not currently being used to link others of theinstrumentation modules 120. A first feedback signal FS1 transmitted viathe first channel 114 of the multiplexed bus establishes a firstfeedback loop between sense instrumentation module 122 and sourceinstrumentation module 124. In the example shown in FIG. 1B, in additionto the link between instrumentation modules 122, 124, a second channel116 of multiplexed bus 110 links sense instrumentation module 126 tosource instrumentation module 128. A second feedback signal FS2transmitted via the second channel 116 of the multiplexed busestablishes a second feedback loop between sense instrumentation module126 and source instrumentation module 128. In another example (notshown), a channel of multiplexed bus 110 different from first channel114 links sense instrumentation module 122 to source instrumentationmodule 128, and a channel of multiplexed bus 110 different from secondchannel 116 links sense instrumentation module 126 to sourceinstrumentation module 124. Additional instrumentation modules 120 (notshown) may be mounted in frame 102, and pairs of them may be linked byrespective channels (not shown) of multiplexed bus 110. Thus, inmeasurement apparatus 100, a feedback loop is established via a channelof multiplexed bus 110 between two instrumentation modules, e.g.,instrumentation modules 122 and 124, mounted in frame 102, as shown inFIG. 1A. Additionally, when additional instrumentation modules, e.g.,instrumentation modules 126 and 128, are mounted in frame 102, multiplefeedback loops are established through respective channels ofmultiplexed bus 110 between pairs of the instrumentation modules.

In some embodiments, instrumentation modules 120 are truly modular,i.e., the instrumentation modules are all physically distinct from oneanother. In other embodiments, multiple instrumentation modules 120 arefabricated on a common substrate, e.g., a printed circuit board, and/orshare a common housing, that constitutes part of measurement apparatus100. In an example, multiple sense instrumentation modules 122 arefabricated on a common substrate and/or share a common housing. Inanother example, multiple source instrumentation modules 124 arefabricated on a common substrate and/or share a common housing. In yetanother example, one or more sense instrumentation modules 122 and oneor more source instrumentation modules 124 are fabricated on a commonsubstrate and/or share a common housing. In some examples in whichmultiple instrumentation modules 120 are fabricated on a commonsubstrate and/or share a common housing, the common substrate or commonhousing occupies a single slot 104 of frame 102. In other examples inwhich multiple instrumentation modules 120 are fabricated on a commonsubstrate and/or share a common housing, the common substrate or commonhousing occupies more than one slot 104 of frame 102. When multipleinstrumentation modules 120 fabricated on a common substrate and/orsharing a common housing are paired, the instrumentation modules arelinked by a feedback signal transmitted via a respective channel ofmultiplexed bus 110.

FIGS. 2, 3, and 4 are block diagrams showing respective examples ofsense instrumentation module 122 and source instrumentation module 124of measurement apparatus 100. Multiplexed bus 110 is additionally shown.FIG. 2 shows an example 140 of sense instrumentation module 122 and anexample 150 of source instrumentation module 124. FIG. 3 shows anotherexample 142 of sense instrumentation module 122 and another example 152of source instrumentation module 124. FIG. 4 shows yet another example144 of sense instrumentation module 122 and yet another example 154 ofsource instrumentation module 124. In the examples shown, the firstchannel 114 of multiplexed bus 110 links sense instrumentation module140, 142 or 144 to source instrumentation module 150, 152, or 154,respectively. In other examples (not shown), a channel of multiplexedbus 110 different from first channel 114 links sense instrumentationmodule 140, 142 or 144 to source instrumentation module 150, 152, or154, respectively. Elements of sense instrumentation modules 142, 144that correspond to elements of sense instrumentation module 140 areindicated using the same reference numerals and will not be individuallydescribed. Elements of source instrumentation modules 152, 154 thatcorrespond to elements of source instrumentation module 150 areindicated using the same reference numerals and will not be individuallydescribed.

The examples of sense instrumentation module 122 and sourceinstrumentation module 124 shown in FIGS. 2-4 are described as follows.Sense instrumentation module 122 and source instrumentation module 124collectively include a circuit path 160 that includes a sense circuit170, a feedback controller 180, a source circuit 190, a sense input 172and a source signal output 194. Sense circuit 170, feedback controller180, and source circuit 190 are coupled, in order, in series, betweensense input 172 and source signal output 194. Sense circuit 170 is togenerate a sense output signal SO representing a sensed parameterdependent on a source signal SS. Feedback controller 180 is to generatea control signal CS in response to sense output signal SO. Sourcecircuit 190 is to generate source signal SS in response to controlsignal CS. Sense instrumentation module 122 and source instrumentationmodule 124 additionally collectively include a communication link 200 incircuit path 160. Communication link 200 includes a bus transmitter 210and a bus receiver 220. Bus transmitter 210 and bus receiver 220 areeach configured to select a channel of the multiplexed bus via which tocommunicate. In the example shown, bus transmitter 210 and bus receiver220 communicate with one another via first channel 114 of themultiplexed bus. In another example, bus transmitter 210 and busreceiver 220 communicate with one another via another channel of themultiplexed bus. In yet other examples, bus transmitter 210 communicateswith the bus receiver (not shown) of a source instrumentation module(not shown) different from source instrumentation module 124 via onechannel of multiplexed bus 110, and bus receiver 220 communicates withthe bus transmitter (not shown) of a sense instrumentation module (notshown) different from sense instrumentation module 122 via anotherchannel of multiplexed bus 110. In all the examples shown in FIGS. 2-4,sense circuit 170 and bus transmitter 210 are parts of senseinstrumentation module 122, and bus receiver 220 and source circuit 190are parts of source instrumentation module 124.

In the example shown in FIG. 2, feedback controller 180 is whollylocated in source instrumentation module 124, and communication link 200links the sense output 174 of sense circuit 170 to the input 182 offeedback controller 180. Examples of feedback controller 180 aredescribed below with reference to FIGS. 11A and 11B. Sense circuit 170generates sense output signal SO in response to a sense input signal SIreceived at sense input 172. In this example, communication link 200receives sense output signal SO output at the sense output 174 of sensecircuit 170, transmits the sense output signal via the first channel 114of multiplexed bus 110, and outputs sense output signal SO to the input182 of feedback controller 180. Thus, in this example, sense outputsignal SO is transmitted as feedback signal FS1. Feedback controller 180generates control signal CS in response to a sense reference SR (FIGS.11A, 11B), and sense output signal SO received from sense circuit 170via communication link 200. Source circuit 190 operates in response tocontrol signal CS received directly from the output 184 of feedbackcontroller 180 to generate source signal SS. Control signal CS generatedin response to feedback signal FS1 causes source circuit 190 to generatesource signal SS such that sense output signal SO (which depends onsource signal SS) generated by sense circuit 170 has a predeterminedlevel or value.

In the example shown in FIG. 3, feedback controller 180 is whollylocated in sense instrumentation module 122, and communication link 200links the output 184 of feedback controller 180 to the control signalinput 192 of source circuit 190. Examples of feedback controller 180 aredescribed below with reference to FIGS. 11A and 11B. Sense circuit 170generates sense output signal SO in response to sense input signal SI.Feedback controller 180 generates control signal CS in response to sensereference SR (FIGS. 11A, 11B), and sense output signal SO receiveddirectly from sense circuit 170 at its input 182. In this example,communication link 200 receives control signal CS output at the output184 of feedback controller 180, transmits the control signal via thefirst channel 114 of multiplexed bus 110, and outputs control signal CSto the control signal input 192 of source circuit 190. Thus, in thisexample, control signal CS is transmitted as feedback signal FS1. Sourcecircuit 190 operates in response to control signal CS received fromfeedback controller 180 via communication link 200 to generate sourcesignal SS. The feedback provided by feedback signal FS1 causes sourcecircuit 190 to generate source signal SS such that sense output signalSO (which depends on source signal SS) generated by sense circuit 170has a predetermined level or value.

In the example shown in FIG. 4, part of feedback controller 180 islocated in sense instrumentation module 144, part of feedback controller180 is located in source instrumentation module 154, and communicationlink 200 links the parts of feedback controller 180. In the exampleshown, feedback controller 180 includes a first part 230 located insense instrumentation module 144 and a second part 240 located in sourceinstrumentation module 154. First part 230 includes a first part input232 connected via the input 182 of feedback controller 180 to the senseoutput 174 of sense circuit 170, and a first part output 234 connectedto the bus transmitter 210 of communication link 200. Second part 240includes a second part input 242 connected to the bus receiver 220 ofcommunication link 200, and a second part output 244 connected via theoutput 184 of feedback controller 180 to the control signal input 192 ofsource circuit 190. Sense circuit 170 generates sense output signal SOin response to sense input signal SI. The first part 230 of feedbackcontroller 180 generates a link signal LS in response to sense outputsignal SO. In this example, communication link 200 receives link signalLS output at the first part output 234 of the first part 230 of feedbackcontroller 180, transmits the link signal via the first channel 114 ofmultiplexed bus 110, and outputs link signal LS to the second part input242 of the second part 240 of feedback controller 180. Thus, in thisexample, link signal LS is transmitted as feedback signal FS1. Thesecond part 240 of feedback controller 180 generates control signal CSin response to link signal LS. Source circuit 190 operates in responseto control signal CS received from feedback controller 180 to generatesource signal SS. The feedback provided by feedback signal FS1 causessource circuit 190 to generate source signal SS such that sense outputsignal SO (which depends on source signal SS) generated by sense circuit170 has a predetermined level or value.

Some examples of measurement apparatus 100 include multiple instances ofsense instrumentation module 140 and multiple instances of sourceinstrumentation module 150 described above with reference to FIG. 2.Other examples of measurement apparatus 100 include multiple instancesof sense instrumentation module 142 and multiple instances of sourceinstrumentation module 152 described above with reference to FIG. 3.Other examples of measurement apparatus 100 include multiple instancesof sense instrumentation module 144 and multiple instances of sourceinstrumentation module 154 described above with reference to FIG. 4. Inthese examples, any one of the source instrumentation modules may belinked by a respective channel of multiplexed bus 110 to any one of thesense instrumentation modules. Other examples of measurement apparatus100 include one or more instances of sense instrumentation module 140and source instrumentation module 150, and one or more instances ofsense instrumentation module 142 and source instrumentation module 152and/or one or more instances of sense instrumentation module 144 andsource instrumentation module 154. In these examples, any one theinstances of sense instrumentation module 140 may be linked to any oneof the instances of source instrumentation module 150, but not to any ofthe instances of source instrumentation modules 152, 154. Similarly, anyone the instances of sense instrumentation module 142 may be linked toany one of the instances of source instrumentation module 152, but notto any of the instances of source instrumentation modules 150, 154, andany one the instances of sense instrumentation module 144 may be linkedto any one of the instances of source instrumentation module 154, butnot to any of the instances of source instrumentation modules 150, 152.

FIGS. 2-4 each show sense instrumentation module 122 and sourceinstrumentation module 124 connected to respective ports of a deviceunder test (DUT) 10, and to respective connectors 112 of multiplexed bus110. Specifically, source circuit 190 outputs source signal SS to DUT 10via source signal output 194. A property, such as voltage, current,frequency, or intensity, of source signal SS output by source circuit190 is determined by control signal CS received directly or indirectlyfrom the output 184 of feedback controller 180. Additionally, aproperty, such as voltage, current, frequency, or intensity, of senseinput signal SI received at the sense input 172 of sense circuit 170depends on source signal SS and a property of DUT 10. Sense circuit 170generates sense output signal SO in response to sense input signal SIreceived at sense input 172.

As will be described in more detail below with reference to FIG. 9A,some examples of sense circuit 170 include a measurement circuit (notshown) to measure a property of a device under test that depends onsource signal SS output by source circuit 190. Examples of sense circuit170 include respective measurement circuits to measure any physical,chemical or biological parameter that is capable of measurement and ofbeing represented by an analog or digital sense output signal SO.Examples of such parameters include voltage, current, electrostaticcapacitance, frequency, intensity, and temperature. Additionally, aswill be described in greater detail below with reference to FIG. 10A,some examples of source circuit 190 include respective signal sources togenerate source signal SS. Examples of the properties of source signalSS that can be determined by source circuit 190 include voltage,current, frequency and intensity.

In communication link 200, bus transmitter 210 additionally includes atransmitter output 214 at which the bus transmitter outputs feedbacksignal FS1 to the channel (e.g., first channel 114) of multiplexed bus110 selected to link sense instrumentation module 122 and sourceinstrumentation module 124, and bus receiver 220 additionally includes areceiver input 222 at which the bus receiver receives feedback signalFS1 from the selected channel of the multiplexed bus.

Multiplexed bus 110 is multiplexed in the sense that multiple feedbacksignals FS1, FS2, etc., share a common information signal path. Examplesof the information signal path include, but are not limited to, aphysical signal path, such as an electrical conductor, e.g., a wire or aprinted circuit board trace, or an optical fiber, or a wireless signalpath, such as a modulated RF carrier or a modulated optical signal. Theinformation signal path is divided into multiple channels, each of whichcarries a respective feedback signal. Instrumentation modules 120 areconfigurable so that any two of them can be linked by a selected channelof the signal path. Two instrumentation modules are linked by assigningthe bus transmitter 210 of one of them and the bus receiver 220 of theother of them to the same unoccupied channel of multiplexed bus 110.Channel assignment can be done, for example, by a user settingrespective channel select controls on the instrumentation modules. Moretypically, channel assignment is done using administration software thatallows the user or higher-level software to identify the instrumentationmodules that are to be linked. Then, in response to the identificationinput, the administration software automatically assigns theinstrumentation modules that are to be linked to the same unoccupiedchannel of the multiplexed bus by transmitting configuration data via aseparate signal path of the multiplexed bus or via a channel of themultiplexed bus assigned for this purpose.

FIG. 5 is a schematic drawing showing an example 300 of multiplexed bus110 implemented with time-division multiplexing. The example ofmultiplexed bus 300 shown links sense instrumentation module 122 tosource instrumentation module 124. Multiplexed bus 300 includes aninformation signal path 310, a clock signal path 312, a frame signalpath 314 and a bus controller 320. Bus controller 320 generates a clocksignal CLK for output on clock signal path 312, and divides the clocksignal by an integer to generate a frame signal (FRAME_N) for output onframe signal path 314. Frame signal FRAME_N includes a periodictransition that indicates the start of each frame. In an example inwhich multiplexed bus 300 has N channels, each channel is a time slothaving a duration 1/N of the period of frame signal FRAME_N.

In sense instrumentation module 122, the transmitter output 214 of bustransmitter 210 includes a feedback signal output 215, a clock signalinput 216 and a frame signal input 217. Transmitter output 214 isconnected to multiplexed bus 300 by a respective connector 112. Feedbacksignal output 215 is connected to output feedback signal FS1 toinformation signal path 310, clock signal input 216 is connected toreceive clock signal CLK from clock signal path 312, and frame signalinput 217 is connected to receive frame signal FRAME_N from frame signalpath 314. In source instrumentation module 124, the receiver input 222of bus receiver 220 includes a feedback signal input 225, a clock signalinput 226 and a frame signal input 227. Receiver input 222 is connectedto multiplexed bus 300 by a respective connector 112. Feedback signalinput 225 is connected to receive feedback signal FS1 from informationsignal path 310, clock signal input 226 is connected to receive clocksignal CLK from clock signal path 312, and frame signal input 227 isconnected to receive frame signal FRAME_N from frame signal path 314.

Bus transmitter 210 and bus receiver 220 are synchronized to clocksignal CLK received from bus controller 320 via clock signal path 312.Information signal path 310 is shared among all the instrumentationmodules 120 connected to multiplexed bus 110. Each linked pair ofinstrumentation modules is assigned its own channel, i.e., a time slothaving a duration 1/N of the period of frame signal FRAME_N. Each bustransmitter, e.g., bus transmitter 210, and each bus receiver, e.g., busreceiver 220, counts the cycles of clock signal CLK to determine thestart of the time slot of the respective channel assigned to it. A bustransmitter to which channel n is assigned transmits feedback signal FSnonly during the time slot assigned to channel n. The bus receiver pairedwith the bus transmitter monitors information signal path 310continuously, but receives and demodulates feedback signal FSn on theinformation signal path only during the time slot assigned to channel n.This way, the bus receiver receives and demodulates the feedback signaltransmitted by the bus transmitter with which it is paired.

In other examples, multiplexed bus 110 lacks frame signal path 314, andbus controller 320 indicates the start of each frame by modulating clocksignal CLK.

In another example, multiplexed bus 110 is implemented withfrequency-division multiplexing. In this, each of the feedback signalsFS1 . . . FSN is transmitted by modulating a respective RF carrier oroptical carrier that is transmitted via information signal path 310. Anyof the common modulation schemes may be used. Each channel ofmultiplexed bus 110 has a respective RF carrier frequency or opticalcarrier wavelength assigned to it. A channel of multiplexed bus 110 isassigned to a pair of instrumentation modules by assigning the RFfrequency or optical wavelength of the channel to the pair. The bustransmitter transmits the feedback signal by modulating the RF signal ofthe assigned RF frequency or the optical carrier of the assigned opticalwavelength, and the bus receiver receives the feedback signal by tuningto the assigned RF frequency or optical wavelength. Frequency-divisionmultiplexing is particularly convenient in an analog system, in whichthe various signals that are transmitted as the feedback signal areanalog signals.

In another example, multiplexed bus 110 is implemented withcode-division multiplexing. In this, each of the N channels ofmultiplexed bus 110 has a respective unique code assigned to it. Each ofthe feedback signals FS1 . . . FSN is transmitted by modulating thefeedback signal with one of the N unique codes, and the coded signal isimposed on information signal path 310. A channel of multiplexed bus 110is assigned to a pair of instrumentation modules by assigning the codefor the channel to which the pair is assigned. The bus transmittertransmits the feedback signal by multiplying the feedback signal by thecode for the channel, and the bus receiver receives the feedback signalby multiplying the signal received from the information signal path bythe code for the channel, which decodes the feedback signal. Variousother coding schemes are known and may be used to transmit N feedbacksignals via fewer than N information signal paths.

For the feedback loop between sense instrumentation module 122 andsource instrumentation module 124 to be stable, the transmission latencybetween sense input 172 and source signal output 194 via multiplexed bus110 should be much smaller than the reciprocal of the loop bandwidth.The transmission latency includes (1) the delay time of source circuit190, (2) any delay time between source signal output 194 and the inputof DUT 10 (FIG. 1A), (3) any delay time within DUT 10, (4) any delaytime between DUT 10 and sense input 172, (5) the delay time of sensecircuit 170, (6) the delay time of feedback controller 180, and (7) thedelay time of communication link 200, including modulation by bustransmitter 210, signal transfer via multiplexed bus 110, anddemodulation by bus receiver 220. In accordance with classical controltheory, considering the transfer function of the whole feedback loopfrom the output 184 of feedback controller 180 to the input 182 of thefeedback controller, the delay time must be smaller than 180 degrees atthe unity gain frequency. When feedback controller 180 includes anintegrator, as is the case when feedback controller 180 includes a PIcontroller or a PID controller, the integrator itself has a 90° phasedelay, so the sum of the remaining delay times must be less than 90°.Assuming that delay times 1, 2, 3, 4, and 5 are negligibly small, thenthe total delay time T_(delay), i.e., the delay time of communicationlink 200 and the delay time of the remainder of feedback controller 180,must be less than 90°. One cycle at a frequency of K Hz corresponds to360°, where K Hz is the loop bandwidth. For the feedback loop to bestable, total delay time T_(delay) should be smaller than(1/K)×(90°/360°), i.e.:

T _(delay)<(1/K)×(¼).

Expressed in words, the total delay time should be less than one-fourthof the reciprocal of the loop bandwidth.

In addition, the bandwidth of each channel of multiplexed bus 110 shouldbe a few times, L, larger than the loop bandwidth K Hz. In an example inwhich the feedback signal is a digital signal, M is the number of bitsper sample transmitted in each channel of multiplexed bus 110, P is thenumber of bits allocated to each channel for communication overhead, Nis the number of channels in multiplexed bus 110, and BR is the bit rateof multiplexed bus 110. Bit rate BR should satisfy the following:

BR>(N*L*K)(M+P) Hz

In an example, L=2. In other examples, L is greater than or less than 2.

In the example shown in FIGS. 1A and 1B, multiplexed bus 110 andinstrumentation modules 120 are mounted in the frame 102 of measurementapparatus 100. In other examples, one or more of the instrumentationmodules 120 of measurement apparatus 100 are paired with one or moreexternal instrumentation modules 130 located externally of measurementapparatus 100. In an example, external instrumentation modules 130 aremounted in another measurement apparatus 106 similar to measurementapparatus 100. In another example, external instrumentation modules 130are stand-alone implementations of the above-described instrumentationmodules 120. FIG. 5 additionally shows an example of multiplexed bus 110that is extended beyond measurement apparatus 100 to a remote location,where measurement apparatus 106 and/or one or more standalone externalinstrumentation modules 130 are located.

In an example (not shown), the physical structure of multiplexed bus 110extends to the remote location and connects to the multiplexed bus ofmeasurement apparatus 106 or to the bus transmitter or bus receiver ofexternal instrumentation module 130. In the example shown in FIG. 5, acommunication interface 330 is connected to multiplexed bus 110 toextend multiplexed bus 110 to the remote location where anothercommunication interface 332 is located. Communication interfaces 330,332 are connected using a suitable electrical, optical or wirelessconnection 334. In various examples, communication interface 330 is auniversal serial bus (USB) interface, an Ethernet interface, an opticalinterface, or a wireless network interface, such as a Wi-Fi or Bluetoothinterface. Communication interface 332 is part of, or is connected to,measurement apparatus 106 and/or the one or more standalone externalinstrumentation modules 130 at the remote location. Any suitablecommunication technique can be used to link one or more of theinstrumentation modules 120 of measurement apparatus 100 to respectiveremotely-located external instrumentation modules 130 that arestand-alone instrumentation modules or part of another measurementapparatus, for example, measurement apparatus 106, provided that thecommunication link established using the communication technique betweenthe linked instrumentation modules satisfies the latency and bandwidthrequirements described above. Measurement apparatus 100, communicationinterface 330 and one or more external instrumentation modules 130collectively constitute a multichannel measurement system 340.

FIG. 6 is a block diagram showing an example of measurement apparatus100 configured to perform a four-terminal (Kelvin) resistancemeasurement. Such a resistance measurement is performed when theconductors supplying current to the device under test have significantresistance, e.g., resistance comparable with that of the device undertest.

In the example shown, measurement apparatus 100 includes a senseinstrumentation modules 122, 126 and source instrumentation modules 124,128. Feedback signal FS1 transmitted from sense instrumentation module122 to source instrumentation module 124 via the first channel 114 ofmultiplexed bus 110 establishes a first feedback loop, and feedbacksignal FS2 transmitted from sense instrumentation module 126 to a sourceinstrumentation module 128 via the second channel 116 of multiplexed bus110 establishes a second feedback loop. The source signal output 194 ofsource instrumentation module 124 is connected to a first terminal 12 ofDUT 10 via a conductor 350 having a significant resistance R_(T), andthe source signal output 194 of source instrumentation module 128 isconnected to a second terminal 14 of DUT 10 via a conductor 352 alsohaving a significant resistance R_(T). The first terminal 12 and thesecond terminal 14 of device under test 10 are additionally connected tothe sense input 172 of sense instrumentation module 122 and the senseinput 172 of sense instrumentation module 126. Source instrumentationmodule 124 and source instrumentation module 128 apply applied voltagesV_(R1) and V_(R2), respectively, to the ends of conductors 350, 352remote from DUT 10. Sense instrumentation module 122 senses a resultingsense voltage V_(N1) at the first terminal 12 of DUT 10 and generatesfeedback signal FS1 in response thereto. In response to feedback signalFS1 received from sense instrumentation module 122, sourceinstrumentation module 124 varies applied voltage V_(R1) to set sensevoltage V_(N1) to a first specified voltage. The first specified voltageis determined by the value or level of sense reference SR (describedbelow with reference to FIGS. 11A, 11B) located in or received by senseinstrumentation module 122 (FIG. 2) or source instrumentation module 124(FIG. 3 or 4). Additionally, sense instrumentation module 126 sensessense voltage V_(N2) at the second terminal 14 of DUT 10 and generatesfeedback signal FS2 in response thereto. In response to feedback signalFS2 received from sense instrumentation module 126, sourceinstrumentation module 128 varies applied voltage V_(R2) to set sensevoltage V_(N2) to a second specified voltage. The second specifiedvoltage is determined by the value or level of sense reference SR in orreceived by sense instrumentation module 126 (FIG. 2) or sourceinstrumentation module 128 (FIG. 3 or 4). As a result of the negativefeedback applied by feedback signals FS1 and FS2, each of sense voltagesV_(N1) and V_(N2) is kept at exactly at its respective specifiedvoltage. Additionally, one of the source modules 124, 128 measures thecurrent output by or received by the respective source module when sensevoltages V_(N1), V_(N2) are at their specified levels. The resistance ofDUT 10 is then calculated by dividing the difference between sensevoltages V_(N1), V_(N2) by the measured current.

FIG. 7 is a block diagram showing an example of measurement apparatus100 configured to measure respective threshold voltages of multiple FETsconcurrently. In the example, only two FETs 20, 30 are shown to simplifythe drawing and the following description. In the example shown,measurement apparatus 100 includes sense instrumentation modules 122,126, and source instrumentation modules 124, 128. Sense instrumentationmodules 122, 126 are current sensing instrumentation modules. Sourceinstrumentation modules 124, 128 are voltage source instrumentationmodules. Measurement apparatus 100 measures a respective thresholdgate-to-source voltage V_(T) that causes each FET 20, 30 to conduct adefined drain current I_(D) with a defined drain-to-source voltageV_(DS) applied. Measurement apparatus 100 additionally includes sourceinstrumentation modules 134, 136, both of which are voltage sourceinstrumentation modules. Source instrumentation module 134 defines thedrain voltages V_(D) and source instrumentation module 136 defines thesource voltages V_(S) of all of the FETs whose threshold voltages are tobe measured concurrently, i.e., FETs 20, 30 in this example. To clarifythe following description, source instrumentation module 134 and sourceinstrumentation module 136 will be referred to as drain voltage (D.V.)module 134 and source voltage (S.V.) module 136, respectively.Specifically, the source signal output 194 of drain voltage module 134is connected to the drains of FETs 20, 30, and the source signal output194 of source voltage module 136 is connected to the sources of FETs 20,30. In another example, measurement apparatus 100 includes a respectiveinstance of drain voltage module 134 and a respective instance of sourcevoltage module 136 for each FET, or a respective instance of drainvoltage module 134 and a respective instance of source voltage module136 for subsets of all the FETs.

In each of sense instrumentation modules 122, 126, sense input 172includes a current input 171 connected to the source signal output 194of drain voltage module 134, and a current output 173 connected to thedrain of FET 20, 30, respectively. Additionally, in each of senseinstrumentation modules 122, 126, the respective bus transmitter 210(FIG. 2, 3 or 4) is set to transmit respective feedback signal FS1, FS2via a different channel of multiplexed bus 110. In each of sourceinstrumentation modules 124, 128, source signal output 194 is connectedto the gate of FET 20, 30, respectively. Additionally, in each of sourceinstrumentation modules 124, 128, the respective bus receiver 220 (FIG.2, 3 or 4) is set to receive the respective feedback signal FS1, FS2from the channel of multiplexed bus 110 via which the respectivefeedback signal was transmitted by sense instrumentation module 122,126, respectively.

Independently of multiplexed bus 110, the respective source circuits 190(FIG. 2, 3 or 4) of drain voltage module 134 and source voltage module136 are instructed to generate the defined drain voltage V_(D) andsource voltage Vs under which threshold voltages V_(T) are to bemeasured and to output these voltages at their respective source signaloutputs 194. Each of sense instrumentation modules 122, 126 measures therespective drain current I_(D) of FET 20, 30, respectively, andtransmits respective feedback signal FS1, FS2 corresponding to themeasured drain current to source instrumentation module 124, 128,respectively. In response to the feedback signal, each of sourceinstrumentation modules 124, 128 varies the gate voltage V_(G) output atits source signal output 194. The feedback process continues until themeasured drain current I_(D) of each FET 20, 30 is equal to a specifieddrain current. The specified drain current is represented by the levelor value of sense reference SR (FIGS. 11A, 11B). Once the measured draincurrent of each FET 20, 30 is equal to the specified drain current, eachsource instrumentation module 124, 128 outputs a level or valuerepresenting the respective gate voltage V_(G) output at source signaloutput 194 of the source instrumentation module as the measuredthreshold voltage V_(T) of FET 20, 30, respectively. A version ofmeasurement apparatus 100 equipped with M instances of senseinstrumentation module 122, M instances of source instrumentation module124, one or more drain voltage modules 134, and one or more sourcevoltage modules 136 can measure respective threshold voltages of M FETsconcurrently, where M is an integer that can be in the tens, thehundreds, or more.

In the example just described, drain voltage module 134 and sourcevoltage module 136 each output a respective defined voltage, and do notreceive a feedback signal from another instrumentation module. In otherexamples, at least one of drain voltage module 134 and source voltagemodule 136 receives a feedback signal from another instrumentationmodule. For example, at least one of drain voltage module 134 and sourcevoltage module 136 receives a feedback signal from anotherinstrumentation module (not shown) that measures the difference betweenthe drain voltage output by drain voltage module 134 and the sourcevoltage output by source voltage module 136, and in which sensereference SR represents a target drain-to-source voltage.

FIG. 8 is a block diagram showing an example of measurement apparatus100 having an implementation 370 of source instrumentation module 124configured to operate in response to respective feedback signalsreceived via different channels of multiplexed bus 110 from two or moresense instrumentation modules. Elements of source instrumentation module370 that correspond to elements of source instrumentation module 150described above with reference to FIG. 2 are indicated using the samereference numerals and will not be described again in detail.

The example of source instrumentation module 370 shown includes amulti-channel bus receiver 380, a signal processor 390, feedbackcontroller 180 and source circuit 190. In the example shown,multi-channel bus receiver 380 has a receiver input 382 connected tomultiplexed bus 110 by a respective connector 112. Each channel ofmulti-channel bus receiver 380 is similar to bus receiver 220 describedabove with reference to FIGS. 2-4. Each channel of multi-channel busreceiver 380 is configured to select any one of the channels ofmultiplexed bus 110, or any one of a subset of the channels, from whichto receive a respective feedback signal. The channels of multi-channelbus receiver 380 are additionally configured to select differentchannels of multiplexed bus 110 from which to receive their respectivefeedback signals. Each channel of multi-channel bus receiver 380includes a respective output at which it outputs a respective feedbacksignal received from multiplexed bus 110. In the example shown,multi-channel bus receiver 380 is a two-channel bus receiver andincludes a first output 384 at which it outputs a feedback signal FS1received via a first channel of multiplexed bus 110, and a second output386 at which it outputs a feedback signal FS2 received via a secondchannel of multiplexed bus 110. In other examples, multi-channel busreceiver 380 has more than two channels and a corresponding number ofoutputs.

Signal processor 390 is interposed between multi-channel bus receiver380 and feedback controller 180. In an example in which the feedbacksignals output by multi-channel bus receiver 380 are analog signals,signal processor 390 is an analog signal processor such as a summingcircuit, a differencing circuit, a multiplying circuit, a dividingcircuit, a filter circuit, or a mixing circuit. In an example in whichthe feedback signals output by multi-channel bus receiver 380 aredigital signals, signal processor 390 is a digital signal processorprogrammed to perform functions similar to those just describeddigitally. Signal processor 390 typically has inputs corresponding innumber to the number of channels of multi-channel bus receiver 380. Inthe example shown, the first output 384 and the second output 386 ofmulti-channel bus receiver 380 are connected to a first input 392 and asecond input 394, respectively, of signal processor 390. Signalprocessor 390 additionally has an output 396 connected to the input 182of feedback controller 180. Signal processor 390 processes the feedbacksignals received by source instrumentation module 370 via respectivechannels of multiplexed bus 110 to generate a single feedback signal forinput to feedback controller 180.

In the example of source instrumentation module 370 shown, multi-channelbus receiver 380 receives a first feedback signal FS1 via one channel ofmultiplexed bus 110 from sense instrumentation module 122, and receivesa second feedback signal FS2 via another channel of multiplexed bus 110from sense instrumentation module 126. Signal processor 390 processesfeedback signals FS1 and FS2 output by multi-channel bus receiver 380 togenerate a feedback signal FS for input to feedback controller 180. Inan example, sense instrumentation module 122 and sense instrumentationmodule 126 receive respective sense input signals SI from respectivelocations in a device under test (not shown) between which adifferential signal exists. In source instrumentation module 370,feedback signals FS1 and FS2 received by signal processor 390 frommulti-channel bus receiver 220380 represent the sense input signals SIreceived by sense instrumentation modules 122, 126, respectively. Signalprocessor 390 subtracts one of the feedback signals FS1, FS2 from theother of the feedback signals to generate single-ended feedback signalFS that is input to feedback controller 180.

Other examples of source instrumentation module 370 include multiplesingle-channel bus receivers (not shown) instead of multi-channel busreceiver 380. Each of the single-channel bus receivers is similar to busreceiver 220. Each of the single-channel bus receivers is configured toselect any one of the channels of multiplexed bus 110, or any one of asubset of the channels, from which to receive a respective feedbacksignal. The single-channel bus receivers are additionally configured toselect different channels of multiplexed bus 110 from which to receivetheir respective feedback signals. An example of source instrumentationmodule 370 having multiple single-channel bus receivers includes aninstance of connector 112 connected to the receiver input of eachsingle-channel bus receiver to connect source instrumentation module 370to multiplexed bus 110. Another example of source instrumentation module370 having multiple single-channel bus receivers includes a singleconnector 112 for connecting source instrumentation module 370 tomultiplexed bus 110. Signals received from the multiplexed bus via thesingle connector are distributed within instrumentation module to thereceiver inputs of the single-channel bus receivers. In yet anotherexample, a subset of the single-channel bus receivers are connected tomultiplexed bus 110 via a shared connector 112.

FIG. 9A is a block diagram showing an example 400 of sense circuit 170that constitutes part of sense instrumentation module 122 (FIG. 2, 3 or4). In the example shown, sense circuit 400 includes a measurementcircuit 410 having a measurement circuit input 412 and a measurementcircuit output 414. Measurement circuit input 412 is connected toreceive sense input signal SI from the sense input 172 of sense circuit170. Measurement circuit output 414 is connected to output sense outputsignal SO generated by measurement circuit 410 to the sense output 174of the sense circuit. Measurement circuit 410 includes circuitryconfigured to measure a parameter of a device under test (not shown)represented by sense input signal SI and to generate sense output signalSO. A property of sense output signal SO represents the parameterrepresented by sense input signal SI. Examples of parameters that can bemeasured by measurement circuit 410 are described above.

Measurement circuit 410 may be a duplication (in some cases, an inferiorduplication) of a sense instrument already in the user's possession, orcommercially available for the user to purchase. Modern senseinstruments typically include an output port at which is output ananalog or digital instrument signal. A property of the instrument signalrepresents a parameter of a device under test (not shown) sensed by thesense instrument. FIG. 9B is a block diagram showing another example 420of sense circuit 170 configured for use with an external senseinstrument that is external to measurement apparatus 100. In the exampleshown, sense circuit 420 is connected to an external sense instrument40. Specifically, sense input 172 of sense circuit 420 is connected toreceive an instrument signal IS from an output port 44 of senseinstrument 40. Sense instrument 40 additionally has an input port 42 atwhich it receives sense input signal SI that represents a parameter of aDUT to be sensed by the sense instrument. Sense instrument 40 measuresthe parameter of the DUT represented by sense input SI and generatesinstrument signal IS in response thereto.

Sense circuit 420 includes an auxiliary input circuit 430 having aninput 432 and an output 434. Input 432 is connected to receiveinstrument signal IS from the sense input 172 of sense circuit 420.Output 434 is connected to output sense output signal SO generated byauxiliary input circuit 430 to the sense output 174 of the sensecircuit. Auxiliary input circuit 430 processes instrument signal IS togenerate in response thereto sense output signal SO that is compatiblewith the sense output signal SO generated by measurement circuit 410described above with reference to FIG. 9A, or that is otherwisecompatible with the feedback controller 180, source circuit 190 andcommunication link 200 of sense instrumentation module 122 and sourceinstrumentation module 124. Examples of the processing performed byauxiliary input circuit 430 include analog-to-digital ordigital-to-analog conversion, resampling, reference changing, dynamicrange modification, and frequency range modification.

FIG. 10A is a block diagram showing an example 500 of source circuit 190that constitutes part of source instrumentation module 124 (FIG. 2, 3 or4). In the example shown, source circuit 500 includes a signal source510 having a signal source input 512 and a signal source output 514.Signal source input 512 is connected to receive control signal CS fromthe control signal input 192 of source circuit 190. Signal source output514 is connected to output source signal SS generated by signal source510 to the source signal output 194 of the source circuit. Signal source510 includes circuitry (not shown) that operates in response to controlsignal CS to generate source signal SS on which a measured parameter ofa device under test (not shown) connected to receive the source signaldepends. Examples of source signals that can be generated by signalsource 510 are described above.

Signal source 510 may be a duplication (in some cases, an inferiorduplication) of a source instrument already in the user's possession, orcommercially available for the user to purchase. Modern sourceinstruments typically include an external control port to receive ananalog or digital instrument control signal that defines a property of asource signal generated by the source instrument. FIG. 10B is a blockdiagram showing another example 520 of source circuit 190 configured foruse with an external source instrument that is external to measurementapparatus 100. In the example shown, source circuit 520 is connected toan external source instrument 50. Specifically, the source signal output194 of source circuit 520 is connected to output an instrument controlsignal IC to an instrument control port 52 of source instrument 50.Source instrument 50 additionally has an output port 54 at which itoutputs source signal SS on which a sensed parameter of a device undertest (not shown) connected to receive the source signal depends.Examples of source signals that can be generated by source instrument 50are the same as those described above.

Source circuit 520 includes an auxiliary output circuit 530 having aninput 532 and an output 534. Input 532 is connected to receive controlsignal CS from the control signal input 192 of source circuit 520.Output 534 is connected to output instrument control signal IC generatedby auxiliary output circuit 530 to the source signal output 194 of thesource circuit. Auxiliary output circuit 530 processes control signal CSto generate in response thereto instrument control signal IC compatiblewith the control parameters of source instrument 50. Examples of theprocessing performed by auxiliary output circuit 530 includeanalog-to-digital or digital-to-analog conversion, resampling, referencechanging, level changing, dynamic range modification, or frequency rangemodification.

FIG. 11A is a block diagram showing an example 600 of a feedbackcontroller suitable for use as feedback controller 180 in theabove-described instrumentation modules 120. Feedback controller 600includes a summing circuit 610 in series with a control circuit 620.Summing circuit 610 includes a summing input 612 connected to receivesense output signal SO from the input 182 of feedback controller 180,and error signal output 614, and a sense reference input 616. Sensereference input 616 is connected to receive sense reference SR. In theexample shown, sense reference SR is received from a sense referenceinput 186 of feedback controller 180 via which the sense reference isreceived from an external sense reference source (not shown). In anotherexample, sense reference SR is received at sense reference input 616from a sense reference source (not shown) internal to feedbackcontroller 180. Summing circuit 610 subtracts sense output signal SOfrom sense reference SR to generate an error signal ES that is output aterror signal output 614.

In an example, control circuit 620 is implemented as aProportional-Integral (PI) control circuit. In another example, controlcircuit 620 is implemented as a Proportional-Integral-Derivative (PID)control circuit. Control circuit 620 includes a control circuit input622 coupled to receive error signal ES from the error signal output 614of summing circuit 610, and a control circuit output 624 connected tooutput control signal CS to the output 184 of feedback controller 180.In some examples, control circuit 620 additionally includes a gaincontrol input (not shown) at which a gain control signal (not shown) isreceived to set the gain of the control circuit so that the feedbackbandwidth can be adjusted.

In some implementations of feedback controller 600, such inimplementations suitable for use in instrumentation module 150 shown inFIG. 2 or in instrumentation module 142 shown in FIG. 3, control circuit620 is located in the same instrumentation module 120 as summing circuit610, and control circuit input 622 of control circuit 620 is directlyconnected to receive error signal ES from the error signal output 614 ofsumming circuit 610. In an implementation suitable for use ininstrumentation modules 144, 154 shown in FIG. 4, summing circuit 610constitutes the first part 230 of feedback controller 180, controlcircuit 620 constitutes the second part 240 of feedback controller 180,and the control circuit input 622 of control circuit 620 is coupled bycommunication link 200 to the error signal output 614 of summing circuit610. In other implementations in which feedback controller 600 isdistributed between two instrumentation modules, the partitioning offeedback controller 600 between first part 230 and second part 240 isdifferent from that just described.

Summing circuit 610 subtracts sense output signal SO received directlyor via communication link 200 from sense circuit 170 from sensereference SR and outputs error signal ES to control circuit 620 directlyor via communication link 200. In the example shown in FIG. 4, errorsignal ES is transmitted to control circuit 620 as link signal LS.Control circuit 620 then generates control signal CS in response to linksignal LS, and outputs the control signal to source circuit 190 viaoutput 184.

FIG. 11B is a block diagram showing another example 630 of a feedbackcontroller suitable for use as feedback controller 180 in theabove-described instrumentation modules 120. Feedback controller 630includes summing circuit 610 and a ramp generator 640. Elements offeedback controller 630 that correspond to elements of feedbackcontroller 600 described above with reference to FIG. 11A are indicatedusing the same reference numerals, and will not be described again here.Ramp generator 640 includes an error signal input 642 at which the rampgenerator receives error signal ES generated by summing circuit 610 bydetermining the difference between sense reference SR and sense outputsignal SO received at the input 182 of feedback controller 180 directlyor via communication link 200 from sense circuit 170. Ramp generator 640additionally includes a control signal output 644 at which a rampgenerator 640 outputs control signal CS to the output 184 of feedbackcontroller 180. Output 184 is coupled to source circuit 190 directly, orvia communication link 200.

Ramp generator 640 additionally includes a START input at which itreceives a digital or analog start signal defining a start level orvalue, a STOP input at which it receives a digital or analog stop signaldefining a stop level or value, a RATE input at which it receives adigital or analog rate control signal that defines a rate at which aramp signal generated by the ramp generator increases or decreases inlevel or value, and a THRESH input at which it receives a digital oranalog threshold signal defining a threshold level or value.

In some implementations of feedback controller 630, such as animplementation suitable for use in instrumentation module 150 shown inFIG. 2 or in instrumentation module 142 shown in FIG. 3, ramp generator640 is located in the same instrumentation module 120 as summing circuit610, and the error signal input 642 of ramp generator 640 is directlyconnected to the error signal output 614 of summing circuit 610. In animplementation suitable for use in instrumentation modules 144, 154shown in FIG. 4, summing circuit 610 constitutes the first part 230 offeedback controller 180, ramp generator 640 constitutes the second part240 of feedback controller 180, and the error signal input 642 of rampgenerator 640 is coupled by communication link 200 to the error signaloutput 614 of summing circuit 610. In other implementations in whichfeedback controller 630 is distributed between two instrumentationmodules, the partitioning of feedback controller 630 between first part230 and second part 240 is different from that just described.

Initially, ramp generator 640 outputs the level or value defined by thelevel or value defined by the start signal as control signal CS, andthen changes (increases or decreases) control signal CS at the ratedefined by the rate control signal until the control signal reaches thelevel or value defined by the stop signal. While generating the rampsignal, ramp generator 640 monitors error signal ES received fromsumming circuit 610. When the absolute value of the error signal fallsbelow the threshold defined by the threshold signal, the ramp generatorstop changing the control signal and holds the control signal at aconstant level or value. The level of control signal CS held by rampgenerator 640 is that which causes source circuit 190 to generate sourcesignal SS at a level or value that makes sense output signal SO equal tosense reference SR.

This disclosure describes the invention in detail using illustrativeembodiments. However, the invention defined by the appended claims isnot limited to the precise embodiments described.

We claim:
 1. A multichannel measurement apparatus, comprising: a frameto accommodate more than two instrumentation modules; a multiplexed busassociated with the frame, the bus comprising more than two connectors,each for connection to a respective instrumentation module; and a senseinstrumentation module and a source instrumentation module mounted inthe frame and electrically connected to respective ones of theconnectors, the sense instrumentation module and the sourceinstrumentation module collectively comprising: a circuit pathcomprising a sense circuit, a feedback controller, and a source circuitcoupled in order in series between a sense input and a source output,the sense circuit to generate a sense output signal representing asensed parameter dependent on a source signal, the feedback controllerto generate a control signal in response to the sense output signal, thesource circuit to generate the source signal in response to the controlsignal; and a communication link in the circuit path, the communicationlink comprising a bus transmitter and a bus receiver each configured toselect a channel of the multiplexed bus via which to communicate; inwhich: the sense input, the sense circuit and the bus transmitter areparts of the sense instrumentation module; and the bus receiver, thesource circuit and the source output are parts of the sourceinstrumentation module.
 2. The apparatus of claim 1, in which thecommunication link is between the sense circuit and the feedbackcontroller.
 3. The apparatus of claim 1 in which the communication linkis between the feedback controller and the source circuit.
 4. Theapparatus of claim 1, in which the communication link is between partsof the feedback controller.
 5. The apparatus of claim 4, in which: thefeedback controller comprises a summing circuit and a control circuit inseries; and the communication link is between the summing circuit andthe control circuit.
 6. The apparatus of claim 1, in which themultiplexed bus comprises a time division multiplexed bus.
 7. Theapparatus of claim 6, in which the multiplexed bus comprises: aninformation signal path, a frame signal path and a clock signal path;and a bus controller to impose a frame signal on the frame signal pathand a clock signal on the clock signal path.
 8. The apparatus of claim1, in which: the multiplexed bus has a transmission latency; theapparatus has a feedback bandwidth; and the transmission latency is lessthan a reciprocal of the feedback bandwidth.
 9. The apparatus of claim1, in which the multiplexed bus comprises a frequency divisionmultiplexed bus.
 10. The apparatus of claim 1, in which the multiplexedbus comprises a code division multiplexed bus.
 11. The apparatus ofclaim 1, in which the feedback controller comprises a ramp generator.12. The apparatus of claim 1, in which: the bus transmitter and busreceiver are each configured to select a first channel of themultiplexed bus via which to communicate; and the apparatus additionallycomprises additional instrumentation modules mounted in the frame andcomprising an additional bus transmitter and an additional bus receivereach configured to select a second channel of the multiplexed bus viawhich to communicate.
 13. The apparatus of claim 1, in which: the bustransmitter and the bus receiver are each configured to select a firstchannel of the multiplexed bus via which to communicate; the sourceinstrumentation module additionally comprises an additional busreceiver; the apparatus additionally comprises an additional senseinstrumentation module mounted in the frame and comprising an additionalbus transmitter, the additional bus transmitter and the additional busreceiver configured to select a second channel of the multiplexed busvia which to communicate; and the feedback controller is located in thesource instrumentation module and comprises a signal processor and acontrol circuit, the signal processor to subject respective feedbacksignals received from the sense instrumentation module and from theadditional sense instrumentation module to an arithmetic operation priorto input to the control circuit.
 14. The apparatus of claim 1, in whichthe sense instrumentation module and the source instrumentation moduleare fabricated on a common substrate.
 15. The apparatus of claim 1, inwhich the sense circuit is to generate the sense output signal from aninstrument signal received from an external instrument, the instrumentsignal representing the parameter sensed by the external instrument. 16.The apparatus of claim 1, additionally comprising a communicationinterface connected to the multiplexed bus.
 17. The apparatus of claim16, additionally comprising one of an electrical, and optical, and awireless communication path connected to the communication interface.18. The apparatus of claim 1, in which the source circuit is togenerate, in response to the control signal, an instrument controlsignal for controlling generation of the source signal by an externalsource instrument.
 19. A multichannel measurement system, comprising: amultichannel measurement apparatus in accordance with claim 1, acommunication interface coupled to the multiplexed bus; an externalinstrumentation module external to the multichannel measurementapparatus, the external instrumentation module linked to one of thesource instrumentation module and the sense instrumentation module viathe communication interface and a channel of the multiplexed bus. 20.The multichannel measurement system of claim 19, in which the externalinstrumentation module is one of (a) an external sense instrumentationmodule to receive an instrument signal from an external instrument, theinstrument signal representing a parameter sensed by the externalinstrument, and (b) an external source instrumentation module to outputan instrument control signal to control generation of an external sourcesignal by an external source instrument.